Due to existing standards, transmission devices of wirelessly operated systems may not exceed a defined transmission power. A typical maximum transmission power is, for example, 10 mW. This corresponds to a maximum transmission level of 10 dBm. Due to this limitation of the transmission power, the range for successful transmission of information is very greatly dependent on the configuration of The receiver device. A problem that has been known for a long time and that it is essential to solve in wireless technology is the construction of receiver devices that feature high sensitivity for the signals to be received, but use little energy themselves for accurate reception of these transmitted signals.
The reception level is greatly damped relative to the transmission level. This damping is caused by the free-space transmission and effects due to interference and the surroundings. Modern wireless systems can definitely reach long ranges, but for this purpose a complicated circuit concept is provided in The receiver devices, wherein these receiver devices themselves require a lot of energy. Examples of complicated circuit concepts are complicated amplifier circuits, correlation receivers, and digital filters with digital computational units.
The decisive factor in receiver technology is the smallest possible reception level at which a signal can still be received accurately in a reception device. The smallest possible reception level is also designated as the sensitivity of The receiver device. The sensitivity is derived from the effective noise power and the necessary minimum signal-to-noise ratio of the reception signal. The noise power is derived, in turn, from the thermal noise-power density N0, the bandwidth B, and the noise figure F of The receiver device. Because the noise-power density of k*T0=4·I0−21 Watt or −174 dBm per Hertz bandwidth and the bandwidth for standard wireless systems are constant, the sensitivity can be increased most effectively by means of the noise figure F of The receiver device. One goal, therefore, is to develop low-noise receiver devices, because these exhibit high sensitivity.
If a receiver device is to be built that has available only a little power-supply energy or that is to consume a little energy for the accurate reception of weak-level signals, then long ranges could not be achieved with conventional circuit concepts. This problem arises especially in energy self-sufficient or battery-operated reception devices that function, for example, according to the energy-harvesting principle. The power-supply energy is obtained from the surroundings.
Reception levels that can be processed and that lie 100 dB or more below the transmission level of 10 dBm are desirable. The receiver device should consume less than a few 100 μW for reception of the signals, in order to be able to use the energy resources for powering The receiver device for as long as possible. Higher energy input necessarily leads to higher energy consumption and thus to faster depletion of the energy resources.
Conventionally, an amplitude-modulated signal is demodulated in The receiver with an envelope-curve detector that has at least one diode in the signal path for rectifying the reception signal. A low-pass filter connected downstream filters the high frequency portions of the received signal. This type of demodulation is very ineffective for the processing of small reception levels. The diodes initially have non-linear characteristic curves. Diodes exhibit low efficiency, in particular in the sub-threshold voltage range. Indeed, in high-frequency technology, specially manufactured diodes are used that feature improved efficiency values, but there are, in addition, parasitic effects. If a demodulator is realized by means of diodes, lower reception levels, typically less than −50 dBm, could no longer be accurately received by The receiver device.
Until now, complicated circuit concepts have been used in order to be able to receive lower reception levels accurately. For example, high-frequency amplifiers are used. These high-frequency amplifiers have a minimum current consumption of a few 10 mA to 100 mA. This current consumption leads to a fast consumption of the energy resources, and is therefore especially undesirable for energy self-sufficient or battery-operated receiver devices.
A desired current-value amplitude that should be received by The receiver without errors is, for example, 1 μA. This corresponds to a reception power level of −90 dBm.
From EP 1 449 309 B1, an activation device for a circuit system that can be controlled remotely is known in which an activation signal is transformed into heat by means of a special heating element. A pyroelectrical element detects the heating of the heating element. By means of an activation circuit connected downstream, a conventional receiver circuit is activated for the reception.
In such an arrangement, it is disadvantageous that time and energy must initially be expended in order to activate the actual receiver device. In addition, the activation circuit presented does not achieve low-energy reception with high sensitivity in The receiver device, but instead merely the listening for whether a signal is present is realized with low energy.